fix: apply history_length for message/send requests (#498)

# Description

changes to honor `historyLength` param passed for `message/send`
requests

Fixes #497  🦕

Release-As: 0.3.9

Author: kthota-g

src/a2a/server/request_handlers/default_request_handler.py

@@ -44,6 +44,7 @@
     UnsupportedOperationError,
 )
 from a2a.utils.errors import ServerError
+from a2a.utils.task import apply_history_length
 from a2a.utils.telemetry import SpanKind, trace_class
 
 
@@ -118,25 +119,7 @@ async def on_get_task(
             raise ServerError(error=TaskNotFoundError())
 
         # Apply historyLength parameter if specified
-        if params.history_length is not None and task.history:
-            # Limit history to the most recent N messages
-            limited_history = (
-                task.history[-params.history_length :]
-                if params.history_length > 0
-                else []
-            )
-            # Create a new task instance with limited history
-            task = Task(
-                id=task.id,
-                context_id=task.context_id,
-                status=task.status,
-                artifacts=task.artifacts,
-                history=limited_history,
-                metadata=task.metadata,
-                kind=task.kind,
-            )
-
-        return task
+        return apply_history_length(task, params.history_length)
 
     async def on_cancel_task(
         self, params: TaskIdParams, context: ServerCallContext | None = None
@@ -363,6 +346,10 @@ async def push_notification_callback() -> None:
 
         if isinstance(result, Task):
             self._validate_task_id_match(task_id, result.id)
+            if params.configuration:
+                result = apply_history_length(
+                    result, params.configuration.history_length
+                )
 
         await self._send_push_notification_if_needed(task_id, result_aggregator)
 

src/a2a/utils/task.py

@@ -70,3 +70,25 @@ def completed_task(
         artifacts=artifacts,
         history=history,
     )
+
+
+def apply_history_length(task: Task, history_length: int | None) -> Task:
+    """Applies history_length parameter on task and returns a new task object.
+
+    Args:
+        task: The original task object with complete history
+        history_length: History length configuration value
+
+    Returns:
+        A new task object with limited history
+    """
+    # Apply historyLength parameter if specified
+    if history_length is not None and task.history:
+        # Limit history to the most recent N messages
+        limited_history = (
+            task.history[-history_length:] if history_length > 0 else []
+        )
+        # Create a new task instance with limited history
+        return task.model_copy(update={'history': limited_history})
+
+    return task

tests/server/request_handlers/test_default_request_handler.py

@@ -836,6 +836,85 @@ async def test_on_message_send_non_blocking():
     assert task.status.state == TaskState.completed
 
 
+@pytest.mark.asyncio
+async def test_on_message_send_limit_history():
+    task_store = InMemoryTaskStore()
+    push_store = InMemoryPushNotificationConfigStore()
+
+    request_handler = DefaultRequestHandler(
+        agent_executor=HelloAgentExecutor(),
+        task_store=task_store,
+        push_config_store=push_store,
+    )
+    params = MessageSendParams(
+        message=Message(
+            role=Role.user,
+            message_id='msg_push',
+            parts=[Part(root=TextPart(text='Hi'))],
+        ),
+        configuration=MessageSendConfiguration(
+            blocking=True,
+            accepted_output_modes=['text/plain'],
+            history_length=0,
+        ),
+    )
+
+    result = await request_handler.on_message_send(
+        params, create_server_call_context()
+    )
+
+    # verify that history_length is honored
+    assert result is not None
+    assert isinstance(result, Task)
+    assert result.history is not None and len(result.history) == 0
+    assert result.status.state == TaskState.completed
+
+    # verify that history is still persisted to the store
+    task = await task_store.get(result.id)
+    assert task is not None
+    assert task.history is not None and len(task.history) > 0
+
+
+@pytest.mark.asyncio
+async def test_on_task_get_limit_history():
+    task_store = InMemoryTaskStore()
+    push_store = InMemoryPushNotificationConfigStore()
+
+    request_handler = DefaultRequestHandler(
+        agent_executor=HelloAgentExecutor(),
+        task_store=task_store,
+        push_config_store=push_store,
+    )
+    params = MessageSendParams(
+        message=Message(
+            role=Role.user,
+            message_id='msg_push',
+            parts=[Part(root=TextPart(text='Hi'))],
+        ),
+        configuration=MessageSendConfiguration(
+            blocking=True, accepted_output_modes=['text/plain']
+        ),
+    )
+
+    result = await request_handler.on_message_send(
+        params, create_server_call_context()
+    )
+
+    assert result is not None
+    assert isinstance(result, Task)
+
+    get_task_result = await request_handler.on_get_task(
+        TaskQueryParams(id=result.id, history_length=0),
+        create_server_call_context(),
+    )
+    assert get_task_result is not None
+    assert isinstance(get_task_result, Task)
+    assert (
+        get_task_result.history is not None
+        and len(get_task_result.history) == 0
+    )
+
+
 @pytest.mark.asyncio
 async def test_on_message_send_interrupted_flow():
     """Test on_message_send when flow is interrupted (e.g., auth_required)."""